1. Field of Invention
This invention relates generally to communication systems and methods. More particularly, this invention relates to communication systems and methods for communicating with a moving vessel.
2. Description of Related Art
Avionics (aviation electronics) systems perform many functions. For both military and civil aircraft, avionics are used for flight controls, guidance, navigation, communications and surveillance. An ever-increasing portion of avionics equipment is being dedicated to communications. Much of the increase comes in the form of digital communications equipment for either digitized voice or data transfer. Military aircraft typically use digital communications to improve security. Civil aircraft use digital communications to transfer data for improved efficiency of operations and radio frequency (RF) spectrum utilization. Regardless of the rationale for implementing digital communication technology, both the civil and military arenas are focusing more on enhanced communications to fulfill the requirements for better operational capability.
The requirements for digital communications for civil aircraft have grown so significantly that the industry, as a whole, has embarked on a virtually total upgrade of the communications systems elements. The goal is to achieve a high level of flexibility in processing various types of information as well as attain compatibility between a wide variety of communications devices. Bandwidth availability poses a special problem for aircraft designers due to weight and electromagnetic interference (EMI) considerations. Generally, a single unit, commonly identified as a communications management unit (CMU), will perform buffering and distribution of the information received by the aircraft. The CMU can receive information via RF transceivers operating in conjunction with terrestrial, airborne, or space-based transceivers. Additionally, it is often advantageous to have several antennas operating on the same vessel or building.
FIG. 1 is a schematic partial cross-sectional view of an aircraft incorporating an antenna system according to the prior art. In FIG. 1, a fuselage 100 of an aircraft 10 bears a right wing 110 and a left wing 120, an upper antenna 140 and a lower antenna 130. Antennas 130 and 140 are placed on opposite sides of the fuselage aircraft, in order to provide additional electrical isolation between them by virtue of the electrical shielding effect of the intervening metal. Antennas 130 and 140 are placed symmetrically about the circumference of the fuselage 100; the top-mounted antenna 140 is the counterpart of antenna 130 which is mounted on the bottom of the fuselage 100.
The fuselage 100 of the aircraft 10 contains a first transceiver 141 electrically coupled to the upper antenna 140 and a second transceiver 131 electrically coupled to the lower antenna 130. The transceivers 141 and 131 and the antennas 140 and 130 are well known in the art. In this example, transceiver 141 and antenna 140 perform VHF data transmission and reception and transceiver 131 and antenna 130 perform VHF voice radio. However, any other types of transceivers and antennas may be used.
The upper antenna 140 is positioned at the top of the fuselage 100, along the vertical axis (Y) of the fuselage 100. The upper antenna 140 transmits a signal having a wavelength .lambda..
The lower antenna 130 is positioned at the bottom of the fuselage 100, on the opposite side of the fuselage 100 with respect to antenna 140, along the vertical axis (Y) of the fuselage 100. As a consequence, the distance between the antennas 140 and 130 along the right side of the fuselage is equal to the distance between the antennas along the left side of the fuselage.
The radio wave transmitted by the antenna 140 clockwise, along the fuselage 100 is shown with dashed lines 160. The radio wave transmitted by the antenna 140 counterclockwise along the fuselage 100 is shown with dashed lines 150. Arrow 170 represents the electric field received by antenna 130 from antenna 140 by radiowaves 150 and arrow 180 represents the electric field received by antenna 130 from antenna 140 by radiowaves 160.
FIG. 2 is a schematic side view of an aircraft incorporating an antenna system according to the prior art. As shown in FIG. 2, the aircraft includes a fuselage 100 and wings 110. In order to reduce the interference level between antennas 230 and 240, the distance between those antennas is maximized. Thus, antenna 230 is located proximate to the nose of the aircraft and antenna 240 is located proximate to the tail of the aircraft.